Adhesion of resin to dentin presents a quite different task from adhesion to enamel. Acid etching can be used to facilitate the adhesion of resin to enamel. However, dentin is attached to the root and there exists moisture. Acid etching can be irritating and cause discomfort. At the same time, dentin fluid may leak out and reduces retentive resin tags, thus hampering the physical adjoinment of the resin and dentin. Therefore, acid etching method cannot be used for the adhesion of resin to dentine. Hence, new development direction of dentin adhesives is directed toward the formation of chemical bonding of resin to the organic and inorganic components of dentin.
The main component of dentin is approximately 75% inorganic compound, of which with hydroxyapatite being the major component, 20% organic material, which primarily consists of collagen, and 5% moisture. A dentin adhesive can provide bonding either with hydroxyapatite (such commercial product as Bondlite, Scotchbond, Dentin Bonding Agent, Clearfil New Bond, Prisma Universal Bond or Scotchbond, etc.) or with organic collagen (such as GLUMA, Dentin Adhesit, etc.) The relative effectiveness of these products is still being studied clinically, each having its respective advantages and disadvantages.
Both of the commercial products of GLUMA and Scotchbond 2 contain 2-hydroxyethyl methacrylate, hereinafter HEMA. The development history, composition, and mechanism of bonding of these two competing products are as follows: